This post is an unexpected sequel to a post I published last month about how single-celled microbes can evolve into multicellular bodies.

Here’s a quick recap of that story. Life became multicellular at least a couple dozen times over the past few billion years. To explore the factors that drove life through these transitions, scientists at the University of Minnesota ran experiments with single-celled yeast. They gave the yeast time to settle in a flask and then drew out some fluid from the bottom. Repeating this many times created conditions in which the yeast quickly evolved into snowflake-like clumps. Bigger clumps fell faster, providing a reproductive advantage over single-celled yeast, which drifted slowly to the bottom of the flask.

Yeast feed on sugar, but they start digesting their meal before it’s even inside them. Their cell walls are loaded with enzymes that break down sucrose into two smaller kinds of sugar, glucose and fructose. The yeast cell can then pump those small sugars into its interior. But a lot of that sugar diffuses away from the cell wall and away from the yeast.

This waste doesn’t matter much when yeast can gorge themselves on a dense soup of sugar. But when sucrose is scarce, losing so much sugar makes it very hard for yeast to grow. The Harvard scientists wondered what course evolution would take if they reared single-celled yeast on a such a meager diet.

They allowed yeast to grow in a flask supplied with only a little sucrose, and then they drew a little fluid to seed a fresh flask. The scientists repeated this for dozens of rounds, and ran a dozen separate trials on different populations of yeast. In eleven trials, the yeast evolved to form clumps.

The clumps typically took on the same snowflake-like structure seen in the University of Minnesota experiment I wrote about last month. And they developed in much the same way. When a cell budded off a new cell, its daughter remained attached rather than making a clean break.

To investigate these clumps, the Harvard scientists put them in a flask with their single-celled ancestors and let them compete for the sucrose. Every time the researchers ran the experiment, the multicellular clumps won, swiftly eliminating their ancestors. Their victory strongly suggests that natural selection was responsible for their evolution to clumps. But their transformation only gave them an advantage when they fed on sucrose. If the scientists added fructose or glucose to their diet, they lost their competitive edge.

The scientists then took a close look at the biochemistry of the evolved yeast. They gained an advantage partly from an improvement in how they fed. The evolved yeast produced more sucrose-digesting enzymes. They also made more proteins to transport the smaller sugars into their interior.

But multicellularity also helped them survive on scarce food. The reason a body provided an advantage has to do with how yeast eat. As yeast cells break down sucrose, they release a lot of sugar into their immediate neighborhood. If another yeast cell is nearby, it can enjoy this meal for free–in other words, without using energy to make the enzymes to prepare the sugar. And if a lot of yeast cells live next to each other, they can collectively create a bigger buffet of sugar that they can all enjoy.

So in the space of a month, we have two studies that see the origin of multicellularity in the same species–but for two separate reasons. In one experiment, the advantage of falling fast provided the push. In another, it was surviving on scarce food.

It stands to reason that bodies might have provided different advantages to animals and plants and other multicellular organisms. For the ancestors of animals, a body might have made them better predators. For the ancestors of plants, a body might have offered them protection from those predators. It’s hard to study those different factors in this major transformation of life, since they took place hundreds of millions of years ago. So it’s a delight to find that scientists can observe different forces at work a single species, right under their noses.

(Update: I corrected the text to indicate the research took place at Harvard, not Oxford. One of the authors moved on from Harvard to Oxford.)

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6 thoughts on “Another Path For Evolving Bodies”

Great story. ” Life became multicellular at least a couple dozen times over the past few billion years.” Can you give a link or reference to this? And how we know it? I always assumed that the uni- to multicellular transition had been a barrier rarely crossed; clearly I was wrong.

[CZ: Here’s Here’s a good review with a great title: “The Evolution of Multicellularity: A Minor Major Transition?”]

I’d like to know why fungi keep on adopting a unicellular lifestyle i.e. become yeasts. Yeasts are highly polyphyletic and keep popping all over the fungal evolutionary tree and my gut feeling is that the yeast state is derived rather than the primitive character state – at least for the dikaryans, which includes most of the species we think of as fungi. It doesn’t take much to make yeast cells clump together (I have myself been able to do it by knocking out single genes) but what makes a fungus turn its back on mycelial growth, fruiting bodies etc and “go it alone” as it were?

Tom: many fungi (such as smut fungi) alternate between a haploid yeast stage and a dikaryotic mycelial stage. As these include some of the most basal groups within Ascomycetes and Basidiomycetes, I suspect this is primitive for Dikarya. Many yeasts (including I think all basidiomycete ones) are simply asexually propagating haploid forms of such fungi.

Lars: I was thinking more of dikaryan fungi that have an almost completely unicellular lifestyle (while perhaps dabbling in some limited hyphal growth or biofilm formation) without the potential to form any mildly advanced structures like fruiting bodies. I mainly have experience with ascomycetous yeasts (several budding and a couple of fission) so my understanding of basidios is a bit shaky. For instance, are species of Cryptococcus, Pseudozyma, Trichosporon, Phaffia etc able to form anything more complex than hyphae or biofilms? I would imagine that they would lose the genes required for forming separate tissues (which to me is the hallmark of a truly multicellular organism) if they no longer require them. Is it known what the minimal gene set is for a fungus to be able to form discrete tissues? And back to my original question – why would some fungi adopt strictly unicellular lifestyles, what would be the advantage? (Some ascomycete yeasts never form hyphae unless you mutate their septation machinery.)

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Ed Yong is an award-winning British science writer. Not Exactly Rocket Science is his hub for talking about the awe-inspiring, beautiful and quirky world of science to as many people as possible.
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